Unfortunately many people & brands have still not taken note that Facebook has fixed New Size for Tabs in a recent development ; from 760px to 520px in width.
How does it matter to you? Well if you are administering a Facebook Fan Page and using the excellent FBML app then you need to resize (or create) your fbml images to max 520px in width and not more than that.
"These updates are designed to simplify navigation for users, reduce complexity for developers, and enable us to build the next generation of tools for growing your business with Facebook."
But I agree far more with Mike Richwalsky on his blog.
Quote - "Said another way: we need the room for ads. That’s my take"
Well, ones gotta do what ones gotta do ;)